1. Field of the Invention
The present invention relates to an apparatus for mixing two fluids, and more particularly to a fluid mixing apparatus for mixing two fluids while passing the two fluids therethrough.
2. Description of the Related Arts
In such devices as a NOx reduction device which reduces NOx contained in an exhaust gas in an internal combustion engine by dosing a reductive gas to the exhaust gas, an EGR device which controls the combustion characteristic of the internal combustion engine by mixing a recirculated exhaust gas into intake air of the internal combustion engine, and a combustor which mixes a gas-turbine fuel and air, a plurality of gases need to be mixed while they pass through the devices. In a diesel engine, for example, NOx is inevitably generated in a thermally efficient, high-temperature combustion. Thus, an aftertreatment is performed in which a reductant is dosed into the exhaust gas to decompose NOx through a reduction reaction. A method of reducing NOx in the exhaust gas by using ammonia, as a reductant, which is generated through a hydrolysis reaction of urea water dosed to the exhaust gas, is called urea-SCR (Selective Catalytic Reduction). The exhaust gas mixed with ammonia is passed through a denitration catalyst so that a denitration reaction (i.e., a reduction reaction of NOx) through ammonia is promoted even at a low temperature. In the denitration reaction of this case, 1 mol of NOx is reacted with 1 mol of ammonia to generate nitrogen and water. It is preferable to perform the mixing such that a molar ratio of NOx to ammonia is 1:1. If the ratio of ammonia is smaller than this ratio, a part of NOx fails to be decomposed due to the shortage of ammonia. As a result, a NOx reduction ratio is decreased. Conversely, if the ratio of ammonia is excessive, ammonia is not used for the denitration. As a result, an excessive portion of the urea water is wasted, and an extra treatment needs to be performed to prevent ammonia from being discharged outside.
Therefore, it is important in the urea-SCR to dose the urea water in appropriate proportion to NOx contained in the exhaust gas such that the ratio of the generated ammonia to NOx is 1:1, and to maintain this ratio even in a local density. That is, the mixing needs to be performed such that a generated ammonia gas is diffused in the exhaust gas at a uniform density.
As another technique of reducing NOx in the diesel engine, there is a technique called EGR (Exhaust Gas Recirculation). According to the EGR, a part of the exhaust gas is mixed with the intake air to increase a ratio of an inactive gas in combustion, and a combustion speed is reduced to suppress local elevation of a combustion temperature. Thereby, the amount of NOx to be generated is reduced. In the EGR, when the exhaust gas is mixed into the intake gas, it is required to suction the exhaust gas while making the pressure of the intake air at a negative value so that the exhaust gas flows toward the intake air even when the pressure of the exhaust gas is low, and to make a mixing ratio of the exhaust gas equal between cylinders so that the mixed exhaust gas has an even density.
As a technique of mixing plural types of gases while conveying the gases, a technique using a swirl flow is commonly known. JP, A 2002-174425 (page 3, FIG. 1) describes a technique of mixing air with a fuel in a gas turbine combustor by swirling the air with a swirl-generating stator and supplying the fuel through a fuel nozzle which has an opening at a downstream side of the swirl-generating stator.